Reverberation generator



y 1956 L. s. GOODFRIEND REVERBERATION GENERATOR Filed June 15, 1950 INVENTOR. m Q? United REVERBERATHUN GENERATGR Lewis S. Goodfriend, New York, N. Y.

Appiication dune 15, 1950, Serial No. 163,346

2 Ciaims. (Cl. 179-4) This invention relates to electronic equipment, and has for its principal object, the provision of a means for modifying electric currents which result from the action of a train of sound waves upon a transducer, so that the resulting currents will, when reproduced, yield superimposed upon the original sound, reverberations of a controlled, and of a controllably varied character.

The quality of sound, as it is heard by a listener, is determined in large measure by two factors; one is the fact that a listener has two ears and that a finite time will elapse between a sound wave train striking first one ear and then the other, causing the listener to hear the same sound twice, each time with a different amplitude and phase; and the other, that the space within which the sound has its origin and is heard will cause the listener to hear not only that train of sound waves which travels from the source to his car by the most direct path, but in addition, a whole series of sounds, each reflected from the boundaries of the confining space, which sounds have longer paths and thus arrive at the listeners ear with amplitudes and phases different from that which arrives by a direct route.

Lack of these reflected sounds, or the inability to hear the same sound twice, strips from the sound a quality, which, for lack of a better characterization, may be called presence, and since, in most reproduced sound, pickup is made by a single microphone close to the source and shielded from extraneous sounds, no multiple path components are present, and the listener is conscious of the consequent lack of presence. The reproduction is mechanical in its sound quality.

This condition is often aggravated by the circumstance that the microphone and sound source are both located in a space whose reverberation characteristic (that is, Whose paths of reflected sound to the microphone from the source and, therefore, the time after which a sound is created in which it drops in amplitude at the point of pickup by said microphone to some arbitrary fraction of its original amplitude at the point of pickup by said microphone) is not suited to the nature of that sound; in which case the sound, as reproduced, loses its true identity in the process of being reproduced, and takes on an unrecognizable and artificial character.

By introducing means for delaying a train of electric waves which have been generated in response to a train of acoustic waves, of delaying said electric wave train by a controlled amount of time, of dividing said delayed wave train and of adding one of said divided trains, after regulating its amplitude, back into the incoming wave train, a means may be provided to produce controlled reverberations of a character which may be varied over wide limits both in amplitude and in phase, and thus a means may be furnished with which a sense of presence may be restored to the reproduced acoustic waves.

In order that this invention may be more clearly understood and more readily carried into effect, reference may be made to the accompanying drawing of a preferred embodiment thereof, and to the description thereof to fitates hatent U Z,?d3,i2 Patented May 2%, 1%56 ice .2 follow, it being specifically understood that said drawing is not intended to limit and define said invention, reference being had to the appended claims for that purpose.

In the drawing:

Fig. 1 is a schematic diagram of the circuits comprising the invention; and

Fig. 2 is a schematic representation of the recording mechanism utilized to effect time delay.

Referring to Fig. 1, the apparatus comprises a transformer 3 whose primary winding 4 is intended for connection to a microphone preamplifier as a source for input current. The resistors 5 and 6, which are shown in association therewith, are included for the purpose of impedance matching.

The secondary 7 of transformer 3 is utilized to develop a voltage between the grid 8 and the cathode 9 of the triode electron tube 10 (which, like others in the drawing and later to be described, is shown as a dual triode, but may be an individual electron tube of triode, tetrode, pentode, or other suitable amplifying structure and characteristic). The capacitor 12 and resistor 13 are the usual bias and bypass resistor and capacitor provided so that the triode 10 will function as a linear amplifier without degeneration.

The amplified output of triode 10, developed across the resistors 14 and 15 which are in series with the plate 16 thereof, is coupled, by way of capacitor 17 from the junction of said resistors, and of lead 18, to the tapped voltage divider comprising resistors i9, 20 and 21, which form, in addition to a stepped attenuator, the grid return for the grid 22 of the electron tube The plate 24 and cathode 25 of electron tube 23 are provided with equal load resistors 26 and 27 in series with said elements, causing said tube to function as a phase inverter, and thus to provide a push-pull driving voltage through the capacitors 28 and 2%, across the resistors 30 and 31 to the grids of the push-pull amplifier tubes 32 and 33.

The electron tubes 32 and 33 share the common bias resistor 34 and bypass capacitor 35, which are connected to a common terminal, and join the cathodes 36 and 37 of said tubes 32 and 33, returning them to ground. Their plates 38 and 39 are connected to the opposite ends of the primary 40 of transformer 41, and energy is supplied thereto (from a 13+ source not shown) by way of the center tap 42 of said primary.

The voltage developed across the secondary 43 of transformer 41, is utilized in a circuit, comprising, in series, the resistor 44, the parallel combination of a variable capacitor and inductor 46, the recording head 47, and the resistor 48. Resistors 44 and 48 are included for the purpose of impedance matching and capacitor 45 and inductor 46 constitute a trap, forming an impedance for preventing the flow of biasing voltage, which is applied to the recording head, from flowing back into and affecting the operation of transformer 41 and its associated elements.

The biasing voltage applied to the recording head 37 is applied by way of the switch 49, enabling said voltage to be turned on and off at will, to the junction of the trap circuit and the recording head 47. This voltage is adjusted to an optimum level by the potentiometer 5%, which is in the output circuit of a buffer amplifier made up of coupling capacitor 51, resistors 52 and 53, and the triode section 54 of the dual electron tube 55. This amplifier, in turn, is driven by an oscillator, which is coupled to the grid 56 of the buffer or isolation amplifier by way of lead 57. The oscillator is made up of the grid leak 58, coupling capacitor 59, tuning capacitor 66, inductor 61, inductor 62, which inductor is mutually coupled to inductor 61, and decoupling filter resistor 63 and capacitor 64 and the triode 65 of the dual electron discharge tube 55. This oscillator supplies a high fre- 3 quency bias voltage which is generally used when recording on magnetic materials.

The combined bias and signal voltages which flow in the recording head 47 are semi-permanently recorded on a magnetic tape 66 which tape, as may be seen from Fig. 2, is driven by a motor 67 and travels in an endless belt over the pulleys 63 and 69. The magnetic information thus recorded will remain invariant without decaying or losing information until intentionally removed. The motor 67 and pulley 68 are movable and may be positioned along the slot 70 in the plate '71. The pulley 63 may also be moved in its slot 72. The recording head 47, however, is fixed to the plate 71 and may not be moved.

After passing over the pulley 69, the tape traverses a pickup head 73 which head is also fixed in position on the plate 71. In passing beneath the recording head 47 the electrical impulses therein are converted into varying magnetic densities along the tape which, upon passing the pickup head, are reconverted into electrical impulses. However, the voltages in the pickup head, although they duplicate the voltages in the recording head, are delayed in time with respect to the recorded voltages by the time required for the tape to travel from the recording head, around the pulley 69 and to the pickup head 73, as driven by the driving motor 67. This time is inversely proportional to the tape velocity and directly proportional to the length of tape therebetween and both of these variables may be adjusted by control of the motor speed by any of the well known means therefor, and by shifting the pulleys in their slots so that the tape length between the recording head and the pickup head is made longer or shorter as required. Multiple heads may be provided with switches so that the same or similar efiects can be produced and idlers, such as the pulleys 74 and 75, are provided so that the tape is kept running smoothly and evenly without slack or whipping.

Referring again to Fig. 1, an electron tube 77 and its associated circuit elements, grid leak 78, capacitor 79, tuning capacitors 8t and 81 and the mutually coupled inductances 82, 83 and 84, together form an oscillator circuit which delivers energy to an erase head 76. The capacitor 85 serves to match the impedance of the erase head 76 to the output impedance of the oscillator. The lamp 86 acts both as a fuse and a means for the dissipation of some of the energy generated by said oscillator.

In Fig. 2 the erase head is designated by the same numeral (76) and is shown to follow the pickup head '73 so that once sensed by the pickup head '73 the varying magnetic densities in said tape are intentionally completely destroyed, and the tape 66, now devoid of any intelligence thereon, is carried around the pulley 63 back to the recording head 4-7 to pick up new information.

The electrical energy obtained from the transformation of the magnetic information on the tape by the action of pickup head 73 is amplified in the amplifier comprising the electron discharge tube 87, shown as a tetrode, with its associated cathode bias resistor 88, cathode bypass capacitor 89, screen dropping resistor 91?, screen bypass capacitor 91 and plate load resistor 92. The amplified energy therefrom is coupled, by way of the capacitor 93 and an equalizing network made up of resistors 94, 95 and 96 and the capacitor 27, to the grid 98 of the triode electron tube 99, where it is again amplified, now by the action of said triode and its associated cathode bias resistor 11, cathode bypass capacitor 1111 and plate load resistor 102.

The signal at this point is identical with the signal input to resistors and 6 but is delayed in time by an amount determined by the tape length and velocity as hereinabove described. This signal is available from the coupling capacitor 1% and is now divided so that the total signal is impressed upon the grid 1 of the triode electron discharge tube 105. This tube, in association with cathode bias resistor 106, cathode bypass capacitor 107 and plate load resistor 1%, acts as an amplifier, delivering its output through the blocking capacitor 1119 to the primary 110 of transformer 111.

A controlled fraction of this voltage is also taken by means of the variable tap 113 on the grid resistor 112 of electron discharge tube 165, and by means of lead 114 separately amplified in the electron discharge tube 115, which triode has associated therewith plate load resistor 116, grid return resistor 117, cathode bias resistor 118 and cathode bias capacitor 119. The amplified output of this triode is coupled by way of capacitor 120 to the junction 121 of the resistors 122 and 123 which resistors, taken together, form a plate load for the electron tube 124. This triode operates as a variable resistor, whose resistance is determined by the voltage divider action of the resistors 125 and 126. With the switch 127 in the position shown in the figure, the resistors 125 and 126 are returned to the B+ source (not shown) at one end and to the ground (or B) at the other. Their junction, by way of resistor 123, which is at a positive potential determined by the setting of the variable tap 131) on resistor 126 is connected to the grid 129 of triode 124. Variation in bias will vary the conductivity of the tube and therefore its resistance. In order to allow for remote operation, a jack 131 is provided so that an external resistor (not shown) may be provided, and the transfer of switch 127 to its alternate contact, the external resistor may be substituted for resistor 126.

The signal voltage is taken at the junction 132 of the resistor 123 and the triode 124, so that the resistor and triode together function as a voltage divider whose ratio is fixed by the setting of the tap 131} of the variable resistor 126 (or the tap on the external resistor if one is plugged in and switched on for operation).

The controlled fraction of voltage, by way of capacitor 134 is applied to the grid 135 of the amplifier comprising triode electron tube 136, grid leak 137, cathode resistor 138, and plate load resistor 139 in series with resistor 15.

Resistor 15 is common to the load resistor of electron tube 10, and hence the input signal is also developed across this same resistor. The output thereacross, therefore, contains both the original signal incoming at that time and the delayed and properly attenuated signal being fed back thereto. These combined signals will now mak their way by the path already outlined of amplifiers and tape record, and thus repeat itself at a constantly decreasing level giving, in etfect, the quality it might have had, had it come to the microphone in which it was originally picked up, by multiple paths, difiering in length and attenuation characteristic.

The disclosed mechanism will, therefore, effectively introduce a reverberation characteristic not originally present in the original signal, of a character which may be varied to modify the same so that the quality of presence may be preserved in the reproduction thereof. This reverberation is determined both by the delay introduced by the tape 66 and the magnitude of the fedback signal on lead 133, both of which may be so manipulated that any desired characteristic may be produced, and the output voltage 140 of the transformer 111 may be made to contain, in addition to the impressed input voltage to resistors 5 and 6, a decaying repetition thereof, controllable in amplitude and rate of decay. The switch 141 is so arranged that the terminals 142 at which this output is available, can be made to match varying load impedances by changing the effective turns ration of the transformer 111.

This system is an improvement over earlier systems in that the signal which is recorded on the magnetic tape remains permanent and invariant until intentionally destroyed. This provides a higher signal to noise ratio than systems in which a fugitive recording or partially erased recording is used. in my system the decaying signal is created by continuously re-recording the original signal at increasingly lower signal levels. The

rate of decay can be controlled by varying the difierence in level between each re-recording of the signal While in systems using fugitive or partially erased recording methods the rate of decay is controlled mainly by the rate of decay of the original recording and the spacing of the pickup heads. Thus a change in apparent room dimension is not achievable by Goldmark by changing the spacing between recording and pickup heads, because he would simultaneously change the extent to which decay had occurred. In my system the apparent room size may be varied by changing the distance between the recording and pickup heads, while independently introducing any desired amount of decay. Thus in my system the apparent room size and the rate of decay are independently controlled which is not the case in earlier systems. My system is therefore an improvement over earlier devices since it provides greater flexibility using fewer and simpler electrical and mechanical components.

What is claimed is:

l. A means for generating reverberations superposed on a train of waves, comprising first amplifier means having a plurality of input means one of which is responsive to said train of waves and a common output means, recorder means responsive to the output of said amplifier means, a semi-permanent magnetic record produced thereby, reproducer means responsive to said record, means coacting with said recording and reproducing means for varying the elapsed time between the recording and the reproduction of said train of waves, second amplifier means having input means and a plurality of output means, said second amplifier input means being responsive to said reproducer means, attenuator means including means for varying the operation thereof locally and remotely responsive to an output of said second amplifier output means and coacting with another of said input means to said first amplifier input means, and output means for said train of waves.

.2. A means for generating reverberations superposed on a train of waves, comprising first amplifier means having a plurality of input means one of which is responsive to said train of waves and a common output means, recorder means having an endless moving magnetic tape and means to vary the speed thereof responsive to the output of said amplifier means, a semi-permanent magnetic record produced thereby, reproducer means responsive to said record, means for positioning said endless magnetic tape to vary the length thereof between said recording and said reproducing means, second amplifier means having input means and a plurality of output means, said second amplifier input means being responsive to said reproducer means, attenuator means including means for varying the operation thereof locally and remotely responsive to an output of said second amplifier output means and coacting with another of said input means of said first amplifier input means, and output means for said train of waves.

References Cited in the file of this patent UNITED STATES PATENTS 1,947,621 Schreiber Feb. 20, 1934 2,105,318 Goldsmith Jan. 11, 1938 2,169,762 Raye Aug. 15, 1939 2,203,353 Goldmark June 4, 1940 2,279,018 Wolfe Apr. 7, 1942 2,286,540 Hanson June 16, 1942 2,322,884 Roetken June 29, 1943 2,420,204 Sinnett May 6, 1947 

